Studying Links via Plats: The Unlink

Abstract

Our main result is a version of Birman's theorem about equivalence of plats, which does not involve stabilization, for the unlink. We introduce the pocket and flip moves, which modify a plat without changing its link type or bridge index. Theorem 1 shows that using the pocket and flip moves, one can simplify any closed $n$-plat presentation of the unknot to the standard 0-crossing diagram of the unknot, through a sequence of plats of non-increasing bridge index. The theorem readily generalises to the case of the unlink.